This application relates to magnetic materials and structures having magnetic tunnel junctions.
A magnetic or magnetoresistive tunnel junction (MTJ) is a multilayer magnetic structure and includes at least three layers: two ferromagnetic layers and a thin layer of a non-magnetic insulator as a barrier layer between the two ferromagnetic layers. The insulator for the middle barrier layer is not electrically conducting and hence functions as a barrier between the two ferromagnetic layers. However, when the thickness of the insulator is sufficiently thin, e.g., a few nanometers or less, electrons in the two ferromagnetic layers can “penetrate” through the thin layer of the insulator due to a tunneling effect under a bias voltage applied to the two ferromagnetic layers across the barrier layer. Notably, the resistance to the electrical current across the MTJ structure varies with the relative direction of the magnetizations in the two ferromagnetic layers. When the magnetizations of the two ferromagnetic layers are parallel to each other, the resistance across the MTJ structure is at a minimum value Rp. When the magnetizations of the two ferromagnetic layers are anti-parallel with each other, the resistance across the MTJ is at a maximum value RAP. The magnitude of this effect is commonly characterized by the tunneling magnetoresistance (TMR) defined as (RAP−Rp) /Rp.
The relationship between the resistance to the current flowing across the MTJ and the relative magnetic direction between the two ferromagnetic layers in the TMR effect may be applied to various applications, including magnetic memory devices and sensor applications. MTJ structures as memory devices store information in the magnetic state of the MTJ and thus are nonvolatile by retaining the stored information when the electrical power is turned off. Magnetoresistive random access memory (MRAM) devices based on the TMR effect, for example, may be an alternative of and compete with electronic RAM devices. For sensing applications, MTJ structures with large TMR values may be used in magnetic read heads to achieve sensitive and high-speed readout of the stored bits and may also be used in various sensor devices.